The present invention relates to multichannel cochlear prosthesis, and more particularly to a multichannel cochlear prosthesis that offers flexible control of the stimulus waveforms.
Cochlear prostheses produce sensations of sound in deaf patients by direct electrical stimulation of the auditory nerve. In modern, multichannel cochlear prostheses, several different sites are stimulated at various distances along the cochlea to evoke the different pitches of sound perception that are normally encoded by nerve activity originating from the respective sites. The patterns of electrical stimulation are derived from acoustic signals picked up by a microphone and transformed by a so-called speech processor that is programmed to meet the particular requirements of each patient. Several different schemes for processing the acoustic signal and transforming it into electrical stimuli have been developed and are well-described in the scientific literature and various patents. All of these schemes can be divided into two basic types on the basis of the waveforms of the electrical stimuli:
i) Analog waveforms, which are essentially filtered versions of the continuous acoustic waveform. Analog waveforms usually involve dynamic range compression, bandpass filtering and scaling to the stimulus current ranges that evoke a satisfactory range of auditory sensations from threshold of perception to maximal comfortable loudness. This produces a rich but poorly controlled set of resultant waveforms. PA1 ii) Biphasic pulses, or more generally, multiphasic pulses, commonly referred to as "pulsatile" waveforms. Biphasic pulses consist of a single cycle of a square wave in which current flows in one direction at a specified magnitude and for a specified brief period of time and is followed immediately by an opposite direction of current of a similar magnitude and duration. Multiphasic pulses comprise a plurality of pulsed waves, i.e., a train of pulses, in which current flows first in one direction, then in another direction, and so on, as required, at specified magnitudes and brief periods of time, in such a way that the charge associated with the total of all the plural pulses is balanced, whereby the net electrical charge delivered to the tissue over one multiphasic cycle is zero. Multiphasic pulses, including biphasic pulses, as used in the prior art are most often delivered in sequence to various sites, with the instantaneous magnitude at each site proportional to some measure of the amount of energy present in a particular frequency band of the acoustic waveform. The result is an impoverished but precisely controlled set of stimulus waveforms.
Both of these stimulus waveform types have been selected because they are relatively easy to produce and modulate electronically for real-time encoding of speech and because they guarantee a charge-balanced alternating current at the electrodes, avoiding net direct current that is known to cause electrolytic damage to both electrodes and body tissues.
Recent findings regarding the complex biophysical phenomena associated with the electrical excitation of neurons and psychophysical phenomena regarding the interpretation of neural activity by the auditory nervous system suggest that the quality and intelligibility of speech percepts evoked by a cochlear prosthesis may be improved in a given patient by more specific manipulations of the electrical stimulus waveforms tailored to that patient. In particular, more complex sequences of polarities, with or without pauses between phases and sites, and with or without simultaneous current delivery at more than one site, appear to be desirable. There is thus a need in the art for a cochlear stimulation system that allows complex stimulation waveforms to be individually tailored for each stimulation site.
The recurring or cyclical manner in which complex or other stimulation waveforms are individually tailored for application to each stimulation site is referred to as a "speech processing strategy". For purposes of the present application, a speech processing strategy thus defines the spatiotemporal manner in which either of the above two types of stimulus waveforms are applied to the cochlea of a patient. The spatial application of the stimulus waveforms is controlled by the type of electrode coupling, e.g., bipolar or monopolar, through which the stimuli are applied to various locations along the inside of the scala tympani duct of the spiral-shaped cochlea. The temporal application of the stimulation waveforms is derived by the timing of the stimuli.
Traditionally, speech processing strategies have thus been classified as either: (1) a simultaneous strategy, or (2) a non-simultaneous, or sequential, strategy. Analog waveforms have traditionally been applied as part of a simultaneous strategy, relying on the more highly focused stimulation provided by bipolar coupling to produce an electrical pattern that will yield speech intelligibility. Short, non-simultaneous, biphasic or multiphasic pulses, on the other hand, are usually applied as part of a sequential speech processing strategy, relying on the highly precise sequence of stimulation pulses through monopolar coupling to produce the electrical pattern that yields speech intelligibility. In a typical sequential, or non-simultaneous, strategy, short biphasic or multiphasic pulses are applied in rapid succession (with little or no time overlap) in a specified pattern to each of multiple channels.
Not all patients benefit from the same speech processing strategy. That is, the complex biophysical phenomena associated with the electrical excitation of neurons and psychophysical phenomena regarding the interpretation of neural activity by the auditory nervous system suggest that the quality and intelligibility of speech percepts evoked by a cochlear prosthesis may be improved in a given patient by more specific manipulations of the electrical stimulus waveforms tailored to that patient. However, heretofore it has been particularly difficult and problematic to manipulate the electrical stimuli in a meaningful manner. There is thus a need in the art for an improved technique or method through which specific manipulations of the electrical stimuli may be tailored and tested by a given patient.
One of the difficulties encountered when trying to formulate or manipulate electrical stimuli in a more meaningful and specific manner has been to efficiently send or transmit all of the needed control data associated with a selected speech processing strategy to the implanted stimulator. This difficulty is compounded when one considers the complexity of, e.g., the multichannel pulsatile speech processing strategies that have recently been developed. A representative description of some of these speech processing strategies may be found in Applicant Faltys' copending U.S. patent application Ser. No. 09/322,712, filed concurrently herewith (Attorney Docket No. AB-072A), which application is incorporated herein by reference. Disadvantageously, the limited bandwidth characteristics of most data information channels established with an implant device have heretofore prevented the efficient transfer of data to the implant device. Here, by "efficient transfer", it is meant a transfer of data at sufficiently low power and at a sufficiently high data rate to allow the data, when received in the implant device, to be effectively used for its intended purpose of defining complex spatiotemporal stimulous patterns on a multiplicity of channels without unduly slowing down the operation of the device. There is thus a need for a more efficient transfer link to an implant device, such as cochlear stimulator.
When a cochlear implant system is first provided to a patient, including implanting any implantable components of the system into the patient (which implantable components typically include at least an electrode and a stimulator, and may also include, for fully implantable systems, a speech processor, a microphone and/or a rechargeable power source), it is necessary to initially "fit" or "adjust" the system. As used herein, it should be noted that the terms "fit", "adjust","fitting" or "adjusting" relate to making electronic or software-programming changes to the system, as opposed to making physical or hardware changes, for the purpose of making the system better perform its intended function of helping the deaf patient to sense sound. Where more than one speech processing strategy is available within the implant system, selecting a speech processing strategy that is best suited for a particular patient thus becomes a key part of the "fitting" or "adjusting" process. Moreover, it should be recognized that the fitting or adjusting process will typically continue to occur at regular or as-needed maintenance/check-up intervals after the initial fitting. Thus, it is seen that where more than one speech processing strategy is available within the implant system, the patient, e.g., through the assistance of his or her physician or audiologist, may receive the benefit of, or at least try out during a trial period, one or more different speech processing strategies than was used initially.
Disadvantageously, not all cochlear implant systems are capable of providing more than one speech processing strategy, or of providing both simultaneous and non-simultaneous speech processing strategies. Rather, most commercially-available cochlear implant systems provide just one type of strategy (simultaneous or non-simultaneous), although some may offer, for example, multiple non-simultaneous strategies. Hence, with these one-type-strategy devices, the patient, and/or the physician/audiologist fitting the patient, has no, or only a very limited, ability to select a suitable speech processing strategy that is most effective for that patient. It is therefore seen that there is a need in the art for a more effective way of making different types of speech processing strategies available to a cochlear implant user.